Research led by a University of Oregon doctoral student in California's Santa Cruz Mountains has uncovered geologic evidence that supports historical narratives for two earthquakes in the 68 years prior to San Francisco's devastating 1906 disaster.
The evidence places the two earthquakes, in 1838 and 1890, on the San Andreas Fault, as theorized by many researchers based on written accounts about damage to Spanish-built missions in the Monterey and San Francisco bay areas. These two quakes, as in 1906, were surface-rupturing events, the researchers concluded.
Continuing work, says San Francisco Bay-area native Ashley R. Streig, will dig deeper into the region's geological record -- layers of sediment along the fault -- to determine if the ensuing seismically quiet years make up a normal pattern -- or not -- of quake frequency along the fault.
Streig is lead author of the study, published in this month's issue of the Bulletin of the Seismological Society of America. She collaborated on the project with her doctoral adviser Ray Weldon, professor of the UO's Department of Geological Sciences, and Timothy E. Dawson of the Menlo Park office of the California Geological Survey.
The study was the first to fully map the active fault trace in the Santa Cruz Mountains using a combination of on-the-ground observations and airborne Light Detection and Ranging (LiDAR), a remote sensing technology. The Santa Cruz Mountains run for about 39 miles from south of San Francisco to near San Juan Batista. Hazel Dell is east of Santa Cruz and north of Watsonville.
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